Medically important parasites are animals that live, take nourishment, and thrive in the body of a host, which may or may not harm the host, but never bring benefit.
They include unicellular organisms such as protozoa (malaria, Giardia, amebiasis, and Cryptosporidium), and helminths (worms), which are categorized as nematodes, cestodes, and trematodes (Box 4.14).
Public health continues to face the problems of parasitic diseases in the developing world. Increasingly, parasitic diseases are being recognized in industrialized countries.
Giardiasis and Cryptosporidium infections in waterborne and other outbreaks have occurred in the United States.
Parasitic diseases such as malaria are among the most common causes of illness and death in the world. Milder illnesses such as giardiasis and trichomoniasis cause widespread morbidity. Intestinal infestations with worms may cause severe complications, although they commonly cause chronic low-grade symptomatology and iron-deficiency
anemia. Deworming every six months has become an effective part of the Expanded Programme of Immunization (EPI plus) along with Vitamin A supplementation and insecticide impregnated bed nets for children.
Echinococcosis
Echinococcosis (hydatid cyst disease) is infection with Echinococcus granulosus, a small tapeworm commonly found in dogs. The tapeworm forms unilocular (single, noncompartmental) cysts in the host, primarily in the liver and lungs, but they can also grow in the kidney, spleen, central nervous system, or in bones. Cysts, which may grow up to 10 cm in size, may be asymptomatic or, if untreated, may cause severe symptoms and even death.
This parasite is common where dogs are used with herd grazing animals and also have intimate contact with humans.
The Middle East, Greece, Sardinia, north Africa, and South America are endemic areas, as are a few areas in
Box 4.14 Neglected Tropical Diseases
“At least 1 billion people suffer from one or more neglected trop-ical diseases (NTDs), such as Buruli ulcer, cholera, cysticercosis, dracunculiasis (guinea-worm disease), food-borne trematode infections (such as fascioliasis), hydatidosis, leishmaniasis, lymphatic filariasis, onchocerciasis, schistosomiasis, soil-trans-mitted helminthiasis, trachoma and trypanosomiasis, although there are other estimates that suggest the number could be much higher. Several of these diseases, and others such as den-gue, are vector-borne. Often, those populations most affected are also the poorest and most vulnerable and are found mainly in tropical and subtropical areas of the world. Some diseases affect individuals throughout their lives, causing a high degree of morbidity and social stigmatization and abuse.
“For a large group of these diseases — mainly helminthic infections — effective, inexpensive or donated drugs are avail-able for their prevention and control. These tools, when used on a large scale, are able to wipe out the burden caused by these ancient scourges of humanity. For leprosy, treatment with effec-tive antibiotics is leading to the elimination of this ancient dis-abling disease. There is also a cost-effective approach to treating yaws that could lead to elimination and final eradication of this debilitating disease that may cause gross deformation. In the case of blinding trachoma, the use of the recommended strategy (SAFE) of an effective antibiotic is enhancing the prog-ress toward final elimination. Large-scale, regular treatment plays a central role in the control of many NTDs such as filariasis, onchocerciasis, schistosomiasis, and soil-transmitted nema-tode infections. For example, regular chemotherapy against intestinal worms reduces mortality and morbidity in preschool children, improves the nutritional status and academic
performance of schoolchildren, and improves the health and well-being of pregnant women and their babies.
“There is a second group of NTDs for which the only clinical option currently available is systematic case-finding and manage-ment at an early stage. These diseases include Buruli ulcer, Chagas’ disease, cholera and other diarrheal diseases, human African trypanosomiasis, and leishmaniasis. Simple diagnostic tools and safe and effective treatment regimens need to be devel-oped urgently for some of these diseases. However, even for these infections, systematic use of the present, imperfect tools at an early stage can dramatically reduce mortality and morbidity. For others, vector control tools are available and present the main method of transmission control, as in the case of Chagas’ disease.
There are examples of great successes in the fight against both of these groups of NTDs. Since 1985, 14.5 million patients have been cured of leprosy through multi-drug therapy; today, less than a million people are affected by the disease. Before the start of the Guinea-worm Eradication Programme in the early 1980s, an estimated 3.5 million people in 20 endemic countries were infected with the disease. In 2005, only about 10,000 cases were reported in 9 endemic countries, and the programme is moving towards eradication. Onchocerciasis has freed more than 25 million hectares of previously oncho-cerciasis-infected land available for resettlement and agricul-tural cultivation, thereby considerably improving development prospects in Africa and Latin America.
Increased awareness and advocacy are needed to draw attention to the realistic prospect of reducing the negative impact of NTDs on the health and social and economic well-being of affected communities.”
Source: World Health Organization. http://www.who.int/neglected_diseases/en/ [accessed October 10, 2007]
the United States and Canada. The human disease has been eliminated in Cyprus and Australia. While the dog is the major host, intermediate hosts include sheep, cattle, pigs, horses, moose, and wolves. Preventive measures include education in food and animal contact hygiene, destroying wild and stray dogs, and keeping dogs from the viscera of slaughtered animals.
A similar, but multilocular, cystic hydatid disease is widely found in wild animal hosts in areas of the northern hemisphere, including central Europe, the former Soviet Union, Japan, Alaska, Canada, and the north-central United States.
Another echinococcal disease (Echinococcus vogeli) is found in South America, where its natural host is the bush dog and its intermediate host is the rat. The domestic dog also serves as a source of human infection.
Surgical resection is not always successful, and long-term medical treatment may be required. Control is through awareness and hygiene as well as the control of wild animals that come in contact with humans and domestic animals.
Control may require cooperation between neighboring countries.
Tapeworm
Tapeworm infestation (taeniasis) is common in tropical countries where hygienic standards are low. Beef (Taenia saginata) and pork (T. solium) tapeworms are common where animals are fed with water or food exposed to human feces.T. solium is especially deadly; delay in diag-nosis and treatment may lead to severe disease, including neurologic cysticercosis. In developing countries, infec-tion is associated with pork consumpinfec-tion, while in the United States, several epidemics have occurred from eating carnivorous game animals such as mountain lions and bears. Freezing or cooking meat, especially that of pigs and carnivorous mammals, is essential to destroy the tapeworm. Fish tapeworm (Diphyllobothrium latum) is common in populations living primarily on uncooked fish, such as Inuit, Eastern European, and Scandinavian.
These tapeworms are usually associated with northern climates.
Toddlers are especially susceptible to dog tapeworm (Dipylidium caninum), which is present worldwide, and domestic pets are often the source of oral–fecal transmission of the eggs. The disease is usually asymptomatic. Similarly, dwarf tapeworm (Hymenolepis nana) is transmitted through oral–fecal contamination from person to person, or via contaminated food or water. Rat tapeworm (Hymenolepis diminuta) also mostly affects young children.
Onchocerciasis
Onchocerciasis (river blindness) is a disease caused by a parasitic worm, which produces millions of larvae that
move through the body causing intense itching, debilitation, and eventually blindness. The disease is spread by a blackfly that transmits the larva from infected to uninfected people. It is primarily located in sub-Saharan Africa and in Latin America, with over 120 million persons at risk.
Control is by a combination of activities including envi-ronmental control by larvicidal sprays to reduce the vec-tor population, protection of potential hosts by protective clothing and insect repellents, and case treatment.
A WHO-initiated program for onchocerciasis control started in 1974 is sponsored by four international agen-cies: the Food and Agriculture Organization (FAO), the United Nations Development Program (UNDP), the World Bank, and WHO. It covers 11 countries in sub-Saharan Africa, focusing on control of the blackfly by destroying its larvae, mainly via insecticides sprayed from the air.
The Vision 2020 program of the WHO aims for control of river blindness by the year 2020.
The program has been successful in protecting some 30 million persons and helping 1.5 million infected per-sons to recover from this disease. WHO estimates that the program prevented 500,000 cases of blindness by 2000 and has freed 25 million hectares of land for resettle-ment and cultivation. The program cost $570 million. This investment is considered by the World Bank to have a return of 16–28 percent in terms of large-scale land reuse and improved output of the population. A WHO program, the African Program for Onchocerciasis Control (APOC), started in 1996, includes Ivermectin and selective vector control efforts by spraying for the blackfly. This involves 30 countries in Africa, and 6 in a similar program in South America (see http://www/who.int/ocp).
Dracunculiasis
Dracunculiasis (Guinea worm disease) is a parasitic disease of great public health importance in India, Pakistan, and central and west Africa. It is an infection of the subcutaneous and deeper tissues caused by a large (60 cm) nematode, usually affecting the lower extremi-ties and causing pain and disability. The nematode causes a burning blister on the skin when it is ready to release its eggs. After the blister ruptures, the worm discharges larvae whenever the extremity is in water. The eggs are ingested in contaminated water and the larvae released migrate through the viscera to locate as adults in the subcutaneous tissue of the leg. Incubation is about 12 months. Larvae released in water are ingested by minute crustaceans and remain infective for as long as a month.
Prevention is based on improving the safety of water supplies and by preventing contamination by infected per-sons. Education of persons in endemic areas to stay out of water sources and to filter drinking water reduces trans-mission. Insecticides remove the crustaceans. Chlorine
also kills the larvae and the crustaceans which prologue larval infectivity. There is no vaccine. Treatment is help-ful, but not definitive.
Dracunculiasis was traditionally endemic in a belt from west Africa through the Middle East to India and central Asia. It was successfully eliminated from central Asia and Iran and has disappeared from the Middle East and from some African countries (Gambia and Guinea).
WHO has promoted the eradication of dracunculiasis.
Major progress has been made in this direction. World-wide prevalence is reported to have been reduced from 12 million cases in 1980 to 3 million in 1990, 152,814 in 1996, and 77,863 cases in 1997. Eradication was antici-pated for 2000; however, the Guinea worm remains endemic in several developing African nations. India’s reported cases fell from 17,000 in 1987 to 900 in 1992, and the country was free from transmission in 1997. In 1997, formerly high-prevalence countries such as Kenya reported no cases in 1997, while Chad, Senegal, Camer-oon, Yemen, and the Central African Republic reported fewer than 30 cases each.
The WHO eradication program was developed suc-cessfully as an independent program with its own direc-tion and field staff, but further progress will require the integration of this program with other basic primary care programs in order to be self-sustaining as an integral part of community health. Community-based surveillance sys-tems for this disease are being converted to work for monitoring of other health conditions in the community.
Schistosomiasis
Schistosomiasis is a parasitic infection caused by the trem-atode (blood fluke) and transmitted from person to person via an intermediate host, the snail. It is endemic in 74 countries in Africa, South America, the Caribbean, and Asia. There are an estimated 200 million persons infected worldwide and more than 600 million at risk for the dis-ease. The clinical symptoms include fever, nausea, vomit-ing, abdominal pain, diarrhea, and hematuria. The organisms Schistosoma mansoni and S. japonicum cause intestinal and hepatic symptoms, including diarrhea and abdominal pain. Schistosoma haematobium affects the genitourinary tract, causing chronic cystitis, pyelonephri-tis, with high risk for bladder cancer, the ninth most com-mon cause of cancer deaths globally. A recently identified species,S. intercalatum, is genetically unique, but thought to cause both intestinal and genitourinary disease.S. inter-calatum is largely identified in inhabitants and immigrants from western Africa. Infection by all schistosomes is acquired by skin contact with fresh water containing con-taminated snails. The cercariae of the organism penetrate the skin, and in the human host it matures into an adult worm that mates and produces eggs. The eggs are disseminated
to other parts of the body from the worm’s location in the veins surrounding the bladder or the intestines, and may result in neurologic symptoms.
Eggs may be detected under microscopic examination of urine and stools. Sensitive serologic tests are also avail-able. Treatment is effective against all three major species of schistosomiasis. Eradication of the disease can be achieved with the use of irrigation canals, prevention of contamination of water sources by urine and feces of infected persons, treatment of infected persons, destruc-tion of snails, and health educadestruc-tion in affected areas.
Persons exposed to freshwater lakes, streams, and rivers in endemic areas should be warned of the danger of in-fection. Mass chemotherapy in communities at risk and improved water and sanitation facilities are resulting in improved control of this disease.
Leishmaniasis
Leishmaniasis causes both cutaneous and visceral disease.
The cutaneous form is a chronic ulcer of the skin, called by various names (e.g., rose of Jericho, oriental sore, and Aleppo boil). It is caused byLeishmania tropica, L. bra-siliensis, L. mexicana, or the L. donovani complex. This chronic ulcer may last from weeks to more than a year.
Diagnosis is by biopsy, culture, and serologic tests. The organism multiplies in the gut of sandflies (Phlebotomus and Lutzomi) and is transmitted to humans, dogs, and rodents through bites. The parasites may remain in the untreated lesion for 5–24 months, and the lesion does not heal until the parasites are eliminated.
Prevention is through limiting exposure to the phlebo-tomines and reducing the sandfly population by environ-mental control measures. Insecticide use near breeding places and homes has been successful in destroying the vector sandflies in their breeding places. Case detection and treatment reduce the incidence of new cases. There is no vaccine, and treatment is with specific antimonials and antibiotics.
Visceral leishmaniasis (kala azar) is a chronic systemic disease in which the parasite multiplies in the cells of the host’s visceral organs. The disease is characterized by fever, the enlargement of the liver and spleen, lymphade-nopathy, anemia, leukopenia, and progressive weakness and emaciation. Diagnosis is by culture of the organism from biopsy or aspirated material, or by demonstration of intracellular (Leishman–Donovan) bodies in stained smears from bone marrow, spleen, liver, or blood.
Kala azar is a rural disease occurring in the Indian sub-continent, China, the southern republics of the former U.S.S.R., the Middle East, Latin America, and sub-Saharan Africa. It usually occurs as scattered cases among infants, children, and adolescents. Transmission is by the bite of the infected sandfly with an incubation period of 2–4 months.
There is no vaccine, but specific treatment is effective and environmental control measures reduce the disease preva-lence. This includes the use of antimalarial insecticides.
In localities where the dog population has been reduced, the disease is less prevalent.
Trypanosomiasis
African Trypanosomiasis (Sleeping Sickness)
Sleeping sickness is a fatal degenerative neurologic dis-ease caused by Trypanosoma brucei, transmitted by the tsetse fly, primarily in the African savannahs, affecting cattle and humans. Subspecies are known to cause both acute and chronic forms of sleeping sickness. Some 55 million persons are at risk in sub-Saharan Africa. Between 1998 and 2004, renewed surveillance and control reduced the incidence of African trypanosomiasis from 38,000 to approximately 18,000. Prevention depends on vector con-trol, and effective treatment of human cases.
Chagas’ Disease (American Trypanosomiasis)
Chagas’ disease is a chronic vector- and blood transfu-sion–borne parasitic disease (Trypanosoma cruzi) which causes significant disability and death. It affects some 17 million persons mainly in Central and South America, with some 300,000 new cases and 45,000 deaths occurring annually. About 30 percent of affected persons develop severe heart disease. While vaccine development is not likely due to the ability of trypanosome antigens to cause autoimmunity and rapid immunologic drift of the organism, two drugs have been developed which show effectiveness in limiting early chronic disease. Brazil achieved elimi-nation of transmission in 1998, after Uruguay (1996) and Venezuela (1997), and followed by Argentina (1999).
While the initial WHO elimination goal by 2010 now seems unfeasible, efforts continue to dramatically reduce the incidence ofT. cruzi infection.
Control is difficult, but control measures include reduc-ing the animal host and vector insect population in its habi-tat by ecological and insecticide measures, education of the population in prevention by clothing, bed nets, and repellents, and with chemotherapy for case management.
Other Parasitic Diseases
AmebiasisAmebiasis is an infection with a protozoan parasite ( Ent-amoeba histolytica) which exists as an infective cyst.
Infestation may be asymptomatic or cause acute, severe diarrhea with blood and mucus, alternating with constipa-tion.E. histolytica infection sometimes results in invasive abdominal infestation, severe liver disease, and death.
Amebic colitis can be confused with ulcerative colitis.
Diagnosis is by microscopic examination of fresh fecal specimens showing trophozoites or cysts. Transmission is generally via ingestion of fecal-contaminated food or water containing cysts, or by oral–anal sexual practices.
Amebiasis is found worldwide. Sand filtration of commu-nity water supplies removes nearly all cysts. Suspect water should be boiled. Education regarding hygienic practices with safe food and water handling and disposal of human feces is the basis for control.
Ascariasis
Ascariasis is infestation of the small intestine with the roundworm Ascaris lumbricoides, which may appear in the stool, occasionally the nose or mouth, or may be coughed up from lung infestation. The roundworm is very common in tropical countries, where infestation may reach or exceed 50 percent of the population. Children aged 3–8 years are especially susceptible. Infestation can cause pul-monary symptoms and frequently contributes to mal-nutrition, especially iron-deficiency anemia. Transmission is by ingestion of infective eggs, common among children playing in contaminated areas, or via the ingestion of uncooked products of infected soil. Eggs may remain viable in the soil for years. Vermox and other treatments are effec-tive. Prevention is through education, adequate sanitary facilities for excretion, and improved hygienic practices, especially with food. Use of human feces for fertilizer, even after partial treatment, may spread the infestation. Mass treatment is indicated in high prevalence communities.
Pinworm Disease (Enterobiasis)
Pinworm disease (oxyuriasis) is common worldwide in all socioeconomic classes; however, it is more widespread when crowded and unsanitary living conditions exist.
The Enterobius vermicularis infestation of the intestine may be asymptomatic or may cause severe perianal itching or vulvovaginitis. It primarily affects schoolchildren and pre-schoolers. More severe complications may occur. Adult worms may be seen visually or identified by microscopic examination of stool specimens or perianal swabs. Transmis-sion is by the oral–fecal ingestion of eggs. The larvae grow in the small intestine and upper colon. Prevention is by edu-cating the public regarding hygiene and adequate sanitary facilities, as well as by treating cases and investigating con-tacts. Treatment is the same as for ascariasis. Mass treatment is indicated in high prevalence communities.
Ectoparasites
Ectoparasites include scabies (Sarcoptes scabiei), the common bed bug (Cimex lectularius), fleas, and lice, including the body louse (Pediculus humanis), pubic louse
(Phthirius pubis), and the head louse (Pediculus humanus capitis). Their severity ranges from nuisance value to seri-ous public health hazard. Head lice are common in school-children worldwide and are mainly a distressing nuisance.
The body louse serves as a vector for epidemic typhus, trench fever, and louse-borne relapsing fever. In disaster situations, disinfection and hygienic practices may be essential to prevent epidemic typhus. The flea plays an important role in the spread of the plague by transmitting the organism from the rat to humans. Control of rats has reduced the flea population, but during war and disasters, rat and flea populations may thrive. Scabies, which is caused by a mite, is common worldwide and is transmitted from person to person. The mite burrows under the skin and causes intense itching. All of these ectoparasites are preventable by proper hygiene and the treatment of cases.
The spread of these diseases is rapid and therefore war-rants attention in school health and public health policy.